1. Field of the Invention
The invention relates generally to mass measuring and more particularly to a weighing system employing a Moire optical device and a readout means for the detection of scale deflection.
2. Brief Description of the Prior Art
In U.S. Pat. No. 3,861,480 a high speed automatic mail processing system for sorting and imprinting postage on large quantities of mixed mail has been described. The system employed an optical readout apparatus for detecting the deflection of the scale tare when supporting a piece of mail for the determination of the postage to be affixed thereon. This prior apparatus included a bank of multiple photodetectors at the end of a light path. A shutter, carried by the tare, was operable to gradually block the light path to successive photodetectors of the bank. A signal indicative of the number of photodetectors remaining in the light path, hence tare deflection, was produced and utilized to set the postage to be imprinted on the piece of mail.
Several disadvantages accompanied the use of this prior optical readout apparatus. For example, numerous photodetectors were required to provide weight indicative signals over a small weight load range with the amount of photodetectors increasing with increased scale capacity; furthermore, the particular transition point wherein each photodetector switched, for example, from conduction to nonconduction states had to be individually set for successive postage weight increments. These transition or switching points were highly critical and tolerance requirements rendered final adjustments difficult and time consuming.
Another disadvantage related to the relatively large projection path, e.g. 18 inches to 24 inches (45-60 cm.), which was required in order to obtain the necessary optical magnification. As a result of such a large projection path and the critical switching points, accuracy of readouts were highly sensitive to vibrations and other environmental conditions.
A similar optical detection system for weight determination is illustrated in U.S. Pat. No. 3,528,517 wherein a coded chart is moved by a load responsive lever with coded markings of the chart being projected upon a bank of photocells. Among the disadvantages of this system were the numerous photocells required for weight determination, with the transition point of each photocell being critical.
In U.S. Pat. No. 3,826,318 a movable and a stationary bar grid in optical alignment were employed. The movable grid functioned as a shutter to sequentially block a light beam through the grids. Electrical signals were generated from the sequential light pulses and were processed to provide a weight indicative signal. A significant drawback of the shutter-grid optical system was that small displacements (less than the spacings between successive bars of the grids) were practically impossible to detect.
The utilization of optical control systems employing moire fringe patterns have been suggested as measurement and control devices for machine tools. Typically, measuring devices such as those illustrated in U.S. Pat. Nos. 2,886,717; 2,886,718; 2,861,345 and 3,154,688, have been designed for utilization in tools such as milling machines in order to measure the movement of the work table of the machine with respect to the framework.
A further position detecting system utilizing moire fringe patterns is disclosed in U.S. Pat. No. 3,755,682. This system requires at least eight photoelectric cells. The output of the photocells are selected in cyclic order by scan pulses to produce a composite output signal, which is compared to a reference signal. The numerous photocells add to the complexity and cost and increase the potential for malfunction.
U.S. Pat. Nos. 3,713,139 and 3,708,681 illustrate typical moire optical devices for determining displacement, and are indicative of one of the drawbacks with prior moire displacement measuring techniques. A plurality of strip photodetectors are employed, each having a length approximating the width of the index grating. In order to synchronize the photodetector spacing with the fringe spacing, the skew angle between the gratings is adjusted to vary the fringe spacings. Unfortunately, minute adjustments in the skew angle result in substantial changes in the fringe spacing, and difficult precise skew angle adjustment is necessary to properly coordinate the fringe patterns with the photodetector spacings.